Such a method and apparatus are know per se. A method for producing dental prostheses is known from German laid open application DE-OS No. 31 18 890, in which a metallic secondary part is anchored removably by means of fastening elements on an also metallic fixed primary part. Bars or friction pins serve as fastening elements, for which corresponding receivers with exact seating are provided in the primary and secondary parts. An erosion machine is used by means of which the seats for the bar in the primary as well as in the secondary parts are produced together by electroerosion. The erosion machine is used only as a drilling tool for producing the seats for the bars. How the primary part is fitted to the tooth stump or the secondary part is fitted to the primary part in regard to the contact surfaces, however, remains completely open.
Further, a method for the production of dental prostheses is known from German laid open application DE-OS No. 33 20 902, wherein a plaster model stump, made electrically conductive and coated and corresponding to the tooth stump formed by the dentist, is utilized as an electrode for the electroerosive finishing of the snug fit of a dental prosthesis made by castings. Until a cementable model of the dental prosthesis made by casting can be produced, five negative-positive transfers are required. The first casting step consists of the making of a negative cast of the natural tooth stump. From this cast a positive plaster model is made. The positive plaster model is supplemented by a wax model made by the dental technician. The positive wax model is embedded in a mold in a secondary cast material and fused. Metal is poured into the casting thus formed and a metal model of the dental prosthesis made in this way.
Each of these casting steps contains errors inherent in the process which partially appear as positive, partially as negative dimensional changes. These dimensional changes are caused, for example, by changes in the volume caused by polymerization of the impression material when making the negative cast. During the production of the plaster model expansion may occur. The processing defects in the wax are the result of the properties of the wax and the working temperature of the wax. The cooling of the wax and setting processes in the embedding material, too, may lead to errors. The working temperature of the liquid cast metal and its change in dimensions during cooling also adversely effect the forming of the metallic model, for example, pores and bubbles are formed and furthermore, structural changes occur which lead to corrosion. The process-related errors add up with each cast leading to considerable dimensional changes which no longer can be accepted and result in considerable finishing work to achieve a snug fit, wherein the cost of this finishing work can amount to as much as half of the total cost of the dental prosthesis.
Another considerable disadvantage of a dental prosthesis made by casting is that only castable materials can be used or that castable materials cannot be used because of a too high melting temperature. For example, work hardened materials could not be used, which are considerably more cost-effective and which exclude from the start the formation of bubbles and structural changes.